The present invention relates generally to an exposure apparatus, and more particularly to a reflection type or catoptric projection optical system, and an exposure apparatus using the same which use ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light to expose objects, such as single crystal substrates for semiconductor wafers, and glass plates for liquid crystal displays (“LCDs”).
Recent demands for smaller and lower profile electronic devices have increasingly demanded finer semiconductor devices to be mounted onto these electronic devices. For example, the design rule for mask patterns has required that an image with a size of a line and space (“L & S”) of less than 0.1 μm be extensively formed. It is expected to require circuit patterns of less than 80 nm in the near future. L & S denotes an image projected onto a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus as a typical exposure apparatus for fabricating semiconductor devices includes a projection optical system for exposing a pattern on a mask or a reticle, onto a wafer. The following equation defines the resolution R of the projection exposure apparatus (i.e., a minimum size for a precise image transfer) where λ is a light-source wavelength and NA is a numerical aperture of the projection optical system:
                    R        =                              k            1                    ×                      λ            NA                                              (        1        )            
As the shorter the wavelength becomes and the higher the NA increases, the higher or finer the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
As a shorter wavelength of light narrows usable glass materials for transmitting the light, it is advantageous for the projection optical system to use reflective elements, i.e., mirrors instead of refractive elements, i.e., lenses. No applicable glass materials have been proposed for the EUV light as exposure light, and a projection optical system cannot include any lenses. It has thus been proposed to form a catoptric projection optical system only with mirrors (e.g., multilayer mirrors).
A mirror in a catoptric reduction projection optical system forms a multilayer coating to enhance reflected light and increase reflectance, but the smaller number of mirrors is desirable to increase reflectance for the entire optical system.
As the EUV exposure apparatus has requires a smaller critical dimension or resolution than a conventional one, higher NA is necessary (e.g., up to 0.2 for a wavelength of 13.4 nm). Nevertheless, conventional three or four mirrors have a difficulty in reducing wave front aberration. Accordingly, the increased number of mirrors, such as six, as well as use of an aspheric mirror, is needed so as to increase the degree of freedom in correcting the wave front aberration. Hereinafter, such an optical system is referred to as a six-mirror system in the instant application. The six-mirror system has been disclosed, for example, in Japanese Patent Applications Publication Nos. 2000-100694 and 2000-235144, U.S. Pat. No. 6,033,079.
Another six-mirror system has been proposed, for example, which intersects light for high NA and possibly reduced mirror's effective diameters (see, for example, Japanese Patent Application Publication No. 2002-006221, and International Publication WO 02/056114A2, and U.S. Pat. No. 5,815,310).
The six-mirror catoptric projection optical system proposed in Japanese Patent Application Publication No. 2002-006221 intersects light from a second mirror M2 to a third mirror with light from a fourth mirror to a fifth mirror so as to reduce mirror's effective diameter, but it has a long span. A small mirror's effective diameter is advantageous to processing and measurements, but a long span and a large volume make it difficult to draw a vacuum in an optical path and to prevent the EUV light from being absorbed in the air.
In addition, an interval between an object surface and a second mirror M2 arranged close to the object surface is very small, such as 50 mm in the first embodiment and 70 mm in the second embodiment. Usually, a reticle is located as an original form of a pattern at an object surface. As this reticle should be exchanged and scanned in exposing a pattern, a stage mechanism should be located near the reticle at sufficiently wide space when the reticle is to be applied to an actual exposure apparatus.
An intermediate image located close to the third mirror M3 causes the energy to concentrate on the third mirror M3, thermally induces aberration, and deteriorates images due to contaminations. Moreover, an arrangement of surface shapes from an object side to an image side, such as concave, concave, concave, concave, convex and concave, has a difficulty in reducing a Petzval sum, and uses only a narrow slit width, such as 0.8 mm.
While a six-mirror catoptric projection optical system proposed in International Publication No. WO 02/056114A2 intersects light from a second minor M2 to a third mirror with light from a fourth mirror to a fifth mirror, a first mirror Ml has such a concave surface that light incident upon the second mirror M2 from the first minor M1 remarkably inclines relative to the optical axis. As a result, a subsequent mirror disadvantageously has a large effective diameter, for example, the fourth minor has a large effective diameter, such as 670 mm. In addition, a long span of 1500 mm has a difficulty in realization in view of processing, measurements, vacuum stability, etc. A large angle between exit light from the first reflective surface and an optical axis also causes the fifth and sixth reflective surfaces to have very large effective diameters. In particular, the effective diameter of the fifth reflective surface is considered to be about 650 mm for NA of 0.25, and a realization becomes difficult in view of a large apparatus and difficult processing and measurements.
While a six-mirror catoptric projection optical system proposed in U.S. Pat. No. 5,815,310 intersects light from a second mirror M2 to a third mirror with light from a fourth mirror to a fifth mirror, a first mirror M1 has such a convex surface that light incident upon the second mirror M2 from the first mirror M1 remarkably inclines relative to the optical axis. As a result, a subsequent mirror disadvantageously has a large effective diameter, such as 500 mm. In addition, an intermediate image located close to the fourth mirror M4 causes the energy to concentrate on the fourth mirror M4, thermally induces aberration, and degrades images due to contaminations.
As the exposure apparatus is usually accommodated in a clean room, and its entire size is limited due to facility restrictions and thus the span of the optical system is limited. In exposure using the EUV light, it is absorbed in the air and the optical path should be made vacuum. Therefore, the size of the optical system is limited from vacuum drawing efficiency. Thus, there should be a sufficient interval between the object surface and (a reflective surface of) a mirror closest to the object surface without increasing the span of the optical system (a distance from the object surface to the image surface) and the effective diameter.
While the catoptric projection optical system in Japanese Patent Publication No. 2000-100694 discloses two embodiments using six-mirror systems with NA of 0.14 and NA of 0.16, the first embodiment with NA of 0.14 is substantially a five-mirror system because the fourth mirror M4 is a plane mirror, which has a difficulty in increasing NA. In addition, the second embodiment with NA of 0.16 uses a spherical mirror for the fourth mirror M4, increasing the degree of design freedom, but requires a distance from the object surface to the image surface is 2 m or greater and has a difficulty in realization.
Either embodiment forms an intermediate image between the second and third mirrors M2 and M3, and arranges four mirrors from the intermediate image to the image surface. Therefore, as a beam width becomes larger with higher NA, a beam enlarges particularly from the intermediate image to the image surface, and has a difficulty in separating mirrors from a beam other than a desired beam and arranging them. Therefore, neither the first embodiment nor the second embodiment can achieve high NA of 0.16 or greater. A compulsory attempt to arrange mirrors would cause another problem to make the maximum effective diameter larger.
Moreover, a distance between the object surface and the mirror M2 closest to the object surface is small, e.g., 20 mm to 30 mm. For example, as shown in FIG. 2, a distance between the second mirror M2 and the mask R is very long. Understandably, it is difficult to apply two optical systems disclosed in Japanese Patent Publication No. 2000-100694 to an actual exposure apparatus.
While an effective diameter of a sixth mirror M6 as a final reflective surface and an optical effective surface enlarge as NA becomes higher, these mirrors should be held with precision. However, the first embodiment extremely narrows an interval between the final reflective surface and a reflective surface closest to a rear surface of the final reflective surface (or an interval between a first mirror M1 and a rear surface of the sixth mirror M6), making holding difficult. Holding becomes more difficult as NA becomes higher. The second embodiment arranges a stop near the first mirror M1, disadvantageously increasing the number of members relating to the stop for adjusting the NA using the variable stop, in addition to hard holding of the final reflective surface. While the second embodiment attempts to expand an interval between a final reflective surface and a reflective surface closest to a rear surface of the final reflective surface (or an interval between a first mirror M1 and a rear surface of the sixth mirror M6), a long distance between the object surface and the image surface, such as 2 m or longer, has a difficulty in realization.
Japanese Patent Publication No. 2000-235144 also discloses catoptric projection optical systems as six-mirror embodiments with high NAs of 0.2, 0.28 and 0.30. Similarly, however, as a distance between the object surface and the mirror M2 closest to the object surface is small, e.g., 80 mm to 85 mm, it is difficult to arrange a stage mechanism for scanning a mask located on the object surface. In addition, it is the fourth mirror M4 that has the maximum effective diameter in either embodiment, and the diameter is large, e.g., 540 mm or greater for NA of 0.2. The largest effective diameter is a diameter larger than 650 mm for NA of 0.28, and the mirror's maximum effective diameter increases simultaneous with high NA. In addition, a distance between a final reflective surface and a reflective surface closest to a rear surface of the final reflective surface (or an interval between a third mirror S3 and a rear surface of a sixth mirror S6) is narrow.
U.S. Pat. No. 6,033,079 discloses two typical six-mirror catoptric projection optical systems, which receive light from the object surface, form an intermediate image via first to fourth reflective surfaces, and re-form the intermediate image on an image surface via a convex fifth reflective surface and a concave sixth reflective surface. Such a structure contributes to high NA by enlarging and introducing light to sixth reflective surface and condensing the entire light on the image surface. Thus, the sixth reflective surface has a large effective diameter. The intermediate-image should be formed after the fourth reflective surface to introduce the light into the fifth reflective surface while preventing the sixth reflective surface from shielding the light.
However, an optical system disclosed in U.S. Pat. No. 6,033,079 has a first convex reflective surface that makes relatively large an angle between exit light from a first reflective surface and an optical axis, and thus a reflective surface at a subsequent stage has a large effective diameter. In this optical system, it is a fourth reflective surface that defines a maximum effective diameter. Since an intermediate image is formed at a position apart from the fourth reflective surface, the fourth reflective surface widely extends and its effective diameter becomes large. As a result, an effective diameter becomes very large, such as 700 mm for NA of 0.25, and a realization becomes difficult in view of a large apparatus and hard processing and measurements. In addition, since the intermediate image is formed apart from the fourth reflective surface and close to a fifth reflective surface, a third reflective surface extends relatively widely. It is more difficult to arrange the third reflective surface, which tends to be located at a position congested with light.
International Publication No. WO 02/056114A2 also discloses a six-mirror catoptric projection optical system. Different from ones disclosed in U.S. Pat. No. 6,033,079, this catoptric projection optical system forms an intermediate image after the second reflective surface and introduces roughly collimated light into the fifth reflective surface.